At present, displays are mainly divided into two types, including thin film transistor-liquid crystal displays (TFT-LCD) and organic light-emitting diode (OLED) displays.
Unlike a TFT-LCD, which controls brightness of display using voltage, an OLED display is current-driven and requires a stable current to control brightness of the OLED. During operation, the OLED display controls the brightness of OLED through a pixel circuit.
For example, a conventional 2T1C (2 transistors and 1 capacitor) pixel circuit comprises a switching transistor T1, a driving transistor T2, and a storage capacitor Cs. A control terminal of T1 is connected to a gate line, a first terminal of T1 is connected to a data line, and a second terminal of T1 is connected to a control terminal of T2. A first terminal of T2 is connected to a supply voltage Vdd, and a second terminal of T2 is connected to an anode of an OLED. A cathode of the OLED is grounded. The storage capacitor Cs is connected in parallel between the control terminal and the second terminal of T2. When scanning of a current pixel is started, and a voltage Vgate provided by the gate line is at a low level (based on an example in which the switching transistor T1 is a P-type transistor), T1 is turned on to write a data voltage Vdata provided through the data line into the storage capacitor Cs. When the scanning ends, Vgate becomes high and T1 is turned off T2 is turned on by the data voltage stored in Cs, thereby driving the OLED to emit light. A driving current of T2, i.e. an operating current of the OLED, can be expressed as IOLED=K(Vgs−Vth)2, wherein Vgs is a gate-source voltage of T2, Vth is a threshold voltage of T2, and K is a coefficient, specifically K=μCoxW/(2L), where p is a carrier mobility, Cox is a gate potential area capacitance, and W and L are a channel width and a channel length of T2, respectively.
Due to the process procedure, device aging and other reasons, the threshold voltages Vths of the driving TFTs of respective pixel points would be different, and voltage drift would be generated with use. As a result, even if the same gate-source voltage Vgs is applied to the driving transistor, the generated current IOLED would vary with Vth, thus affecting the uniformity of display.